This invention concerns an illumination method and device.
Broadly-speaking the invention relates to a method whereby bright illumination provided by a concentrated, narrow beam of light or other electromagnetic radiation can apparently be disseminated, with comparable intensity, over a much wider area. The invention moreover also concerns a light disseminator device which is a combination of light deflector(s) with other means and which is able, in co-operation with a light source, to provide relatively high-intensity apparent illumination over a widespread target area, that is to say wide-arc illumination apparently more intense than could be spread over the same target area by the light source unaided by the device.
It is a commonplace that light emanating from a light source will normally be radiated therefrom broadcast in all directions, with correspondingly low intensity in any one direction. It is however also one of the most basic achievements of optics that light emanating from such a light source can be concentrated and directed by means of a suitable reflector (thus a mirror or system of mirrors) and/or refractor (thus a lens or system of lenses) into a narrow beam, which casts illumination of relatively much greater intensity in a chosen direction than would otherwise have been broadcast in that directionxe2x80x94but of course at the expense of diminishing or denying illumination in other directions. It seems that one is faced with an apparently inescapable choicexe2x80x94between relatively low-intensity illumination over a wide area on the one hand, or relatively high-intensity illumination over a narrow area on the other. And this is indeed the inescapable choice, when the intensity of illumination is perceived entirely objectivelyxe2x80x94there is no avoiding the laws of science, and one does not get something for nothing.
It is known, however, that the perceived intensity of illumination is in certain circumstances not objective but can be quite subjective. This phenomenon is called persistence of vision, and refers to how the human eye can be fooled into perceiving continuous illumination even if it is in fact discontinuous, i.e. rapidly repeated flashes of illumination. Therefore it is possible to produce in the eye of an human (or animal) perceptor an illusion of wide-arc, relatively high-intensity apparent illumination if a narrow, concentrated beam of such relatively high-intensity illumination is intermittently but repeatedly swept at sufficiently high frequency across a wide target area.
Various methods of overcoming the objective problem, which utilise this phenomenon, have been suggested, and the most pertinent of these have been outlined below.
U.S. Pat. Nos. 3,865,790 and 4,153,926 disclose methods and devices which have tried, with only partial success, to solve the problem by taking a device that produces a beam of light, and then rotating the entire assembly at high speed. Similarly British Patents No. 694,357 and No. 1,083,492 both also relate to devices where the light source and the beam concentrating means are rotated together.
Whilst fine in concept, this type of device is rather lacking in practical feasibility. For a start the beam produced tends to be a disc in overall configuration and this is not by any means ideal. The source will only cast light on a given point once (per beam that is produced) per revolution of the source. More importantly however, in devices of this general type, the light source can be one of relatively high power and therefore produce several beams, or it may be confined to producing one beam only and therefor require a less powerful source of light. Naturally when more beams than one are produced, and are able to scan across the target area, then the speed of rotation of the source can be reduced, but even so it will still be required to rotate at high speed. One is faced with the dilemma that if the amount of beams produced is increased, then the speed of rotation can be decreased but the size of the device that must be rotated is increasedxe2x80x94whereas conversely the opposite of course is true in that the size of device can be kept down by using fewer beams, but then the speed at which the device must spin is dramatically increased.
These considerations mean that any design of this type must be fairly cumbersome to contain all the features required to rotate a large and complex object at high speed. For instance it requires fairly complex, and hence unreliable, wiring mechanisms to electrically link the rotating bulb to the power supply. Additionally the whole rotating part must be carefully balanced to prevent vibration and the problems associated with it.
The most important point is however that, during high speed rotation, the filament of the bulb can be forced out of alignment with the optics, due to the centrifugal forces. This is hard to avoid because a filament must by design be of narrow diameter and hence flexible.
In an attempt to overcome some of the problems associated with the above disclosed methods, devices wherein the light source was held stationary and the beam producing means were rotatable therearound were instead proposed. In British Patent No. 488,616 a device with lens arrays rotating about a light source was disclosed. Additionally in British Patent No. 520,079 a fixed light source with a set of rotating parabolic mirrors located around it was proposed. Both these devices suffer from the problem of having to rotate the beam means around the light source at high speed, but in close proximity to the bulb. This is especially a problem of the device of GB 520,079 which had at least two back-less parabolic reflectors joined around the light such that they projected at least two beams of light from the source. This has the effect of producing a weak source of light so that the overall lighting phenomenon is diminished.
Various other methods have been employed in an attempt to achieve the proposed objectives, and they have for example, involved a large rotating tower with complex internal reflectors as in GB 558,828; or they have used vibrating mirrors, light source and rotating prisms to scan light over a small area as in GB 951,604.
All the above have failed to effectively overcome the problems associated with attempting to achieve the objectives of the present invention, or indeed for that matter the objectives they set themselves. Indeed the very fact that none of them ever caught on, gives testament to their lack of effectiveness. The present invention, on the other hand, provides a convenient and effective means of achieving those objectives and overcoming the problems.
Therefore, according to this invention in its broadest aspect, there is provided a method of furnishing a perceptor with apparently-continuous illumination by electromagnetic radiation to which the perceptor is responsive over an extended target area, in which a rotatable reflector is used to deflect a relatively narrow beam of radiation from one point to another over a relatively wide target area, whereby at any instant only part of said area is illuminated with said radiation but every part thereof is intermittently and repeatedly illuminated by discontinuous flashes of said radiation, said flashes being as regards any one part of said target area repeated at time intervals not less than the decay-period of the response of the perceptor to that radiation.
The terms xe2x80x9cradiationxe2x80x9d and xe2x80x9creflectorxe2x80x9d used above, and hereinafter employed for convenience, refer respectively to any suitable electromagnetic radiation that may be efficiently reflected, and to a reflector capable of reflecting said radiation.
It is currently envisaged that the electromagnetic radiation employed will be in the ultraviolet, visible and/or infrared ranges, thus corresponding to wavelengths of say from 1 nm up to about 5 nm. For the purposes at present contemplated it will be preferable to use visible light with wavelengths in the range of from about 380 nm up to about 780 nm, and/or actinic radiation i.e. light in the violet and ultra-violet regions of the spectrum which will bring about chemical or photochemical changes, and may be regarded as corresponding to wavelengths of from 4 to 600 nm. Of course the term xe2x80x9cultra-violet (or UV) radiationxe2x80x9d refers to the non-visible part of actinic radiation, and may be regarded as corresponding to wavelengths of from 4 to 400 nm., and more especially 325-365 nm. Thus overall the preferred visible and actinic radiation for use in the method of the invention corresponds to wavelengths in the range of from 4 nm up to 780 nm. The electromagnetic radiation employed may be coherent, subject to the normal considerations governing its generation and use; but as currently envisaged will usually be normal, incoherent radiation.
Where the context so allows, the term xe2x80x9cperceptorxe2x80x9d as used herein includes not only the human (or other animal) eye responsive in the visible light range but also non-animal (e.g. electric and/or electronic) perceptor instruments responsive in the visible and/or the non-visible radiation ranges. It moreover also includes part-human (or other animal) and part-instrumental perceptors, as for instance when non-visible radiation is perceived initially by an instrument responsive thereto but then converted within that instrument into a secondary image in the visible light range and thus perceptible by the human (or other animal) eye of an ultimate observer.
The decay of the response of any perceptor will generally be exponential, and of course the term xe2x80x9cdecay-periodxe2x80x9d is not here used in an extreme theoretical sense which could include almost infinite periods as the response approaches zero but in its practical sense which embraces only perceptor-responses that are useful for their intended purpose. On an admittedly arbitrary basis the outside limit of the relevant decay-period can be defined as that over which the response of the perceptor falls to 30% of the maximum response of the perceptor to stimulation by that radiation. For all currently-envisaged purposes the decay-period should be set at that during which the perceptor-response falls to no less than 50% of maximum, and it is believed that the best results will be achieved when the relevant decay-period is set to end at a level of 80% or even 90% of maximum response.
In order to reduce or avoid any sensation in the perceptor of flickering in the perceived illumination it is quite desirable that the flashes of illumination should be repeated as regards any one part of the target area at least twice during the decay period, and (within experience so far) it is best if they are repeated substantially three times during that period. When the illumination is in the visible range and the intended perceptor is the human eye these preferences correspond roughly with the flashes of visible light being desirably repeated at least twice every one-tenth of one second, and best repeated substantially three times every one-tenth of one second.
According to another preferred aspect of this invention there is also provided a light disseminator, for use in carrying out the method herein disclosed, which comprises means operable to direct a beam of light so that it impinges upon a rotatably-mounted light-deflector, said light-deflector being arranged and disposed so that dependent upon its rotational position it will deflect the light-beam to one point or another around an arcuate target area centred upon the rotatable deflector, and means operable to rotate the light-deflector so that it sweeps the deflected beam around said arcuate target area, at a rotational rate such that any given part of the arcuate target area is intermittently but repeatedly illuminated by discontinuous flashes of light provided by the deflected light-beam at time-intervals of not more than one-tenth of one second.
In this case, the perceptor is to be the human eye, and the time-intervals should preferably be not more than one-thirtieth of one second, and possibly or even desirably still less.
Of course, the beam-directing means will desirably be so disposed and arranged as normally to direct a beam of substantially parallel light to impinge upon the rotatably-mounted mirror, but it is for some end-uses advantageous also to provide means for adjusting the arrangement out of its normal disposition so as either to converge or to diverge the otherwise substantially parallel light-beam.
The beam-directing means preferably will comprise means for mounting a light-source, and a concave reflector mounted adjacent to said light-source on its side remote from the light-deflector so as to assist in directing the desired parallel light-beam to impinge upon the light-deflector(s).
Alternatively or in addition the beam-directing means may comprise means for mounting a light-source, and a convex lens or lens system mounted between said light-source and the light-deflector so as to assist in directing the desired parallel light-beam to impinge upon the light-deflector(s).
The light-disseminator will normally include an electrically-operable incandescent light-source supported in the mounting means, and there provided with electrical connections adapted under control to operate the incandescent light-source. The light-source advantageously is or includes a single-filament incandescent light bulb so supported in the mounting as to dispose the filament with its axis normally vertical.
The light-deflector may be a refractor, e.g. a multi-sided-prism, but experience so far suggests that it is advantageously a rotatably-mounted reflector, usually indeed a multi-faceted reflector. For the purposes currently envisaged the rotational axis of the light-deflector(s) should in normal use be disposed vertically.
In the simplest arrangement the multi-faceted reflector will advantageously be a double-side plane mirror. With such an arrangement, and in an ideal set-up wherein a beam of truly parallel light from a truly linear source is incident upon a plane mirror of the same depth as the beam, then the reflected beam will be neither divergent nor convergent, and thus will have the same depth as the incident beam. Therefore on rotation of the mirror the reflected beam will be swept around a substantially 360xc2x0 arc, creating at any given instant a corresponding small patch of high-intensity illumination, (having the same depth as both the incident beam and the linear source) at one particular point on the 360xc2x0 arc centred on the rotating mirror. In practice it is however effectively impossible to achieve such an ideal set-up, and there is an inevitable tendency for the beam incident on the mirror to include some stray, non-parallel lightxe2x80x94and in that event the beam even when reflected from a plane mirror will to some extent be slightly divergent. Nevertheless when using a beam of parallel light and a plane mirror most of the light is concentrated in the previously-mentioned small patch, and due to persistence of vision in an human observer""s retina it will be perceived as a fairly thin, flat xe2x80x9cbandxe2x80x9d of illumination around the rotating mirror, so-to-speak in a sort of horizontal disc.
Dependent upon requirements, it is possible either to accentuate the tendency for the beam to diverge or to try to counteract it.
Thus, in order to promote a wider band of illumination the light-deflector can be so constructed and arranged that it encourages the substantially-parallel light-beam impinging thereon to become divergent in the vertical planes containing the rotational axis of the light-deflector, e.g. by making the light-deflector a slightly-convex mirror.
Conversely, if it should be wished to concentrate the illumination into a still narrower band, then the light-deflector can be so constructed and arranged that it counters any tendency for the substantially-parallel light-beams impinging thereon to become divergent, or indeed even forces it to become convergent, e.g. by making the light-reflector a slightly-concave mirror.
The transverse dimensions of the light-deflector in the plane normal to the impinging light-beam will desirably exceed the width of that light-beam, so as to ensure that the full width of the light-beam is deflected thereby for so much as possible of its rotation. On the other hand the light-deflector would have to be of infinite width if it were to be capable of deflecting the full width of the incident light beam throughout its entire rotation, which of course is absurdly impossible.
Balancing these considerations, it currently appears that for practical purposes the width of the light-deflector (normal to the incident beam, and in the plane normal to its rotational axis) should conveniently be in the range of from about 1.12 to about 2.24 times the width of that beam. On a somewhat arbitrary basis, it is currently thought best if the width of the light-deflector is substantially 1.4 times the width of the beam.
The light disseminator of this invention may be embodied in various ways according to the end-use envisaged. Possible uses seem very extensive, and have not yet been fully explored, but fall broadly into two categories. In one category of end-use the ultimate observer carries the device himself or for instance upon a vehicle, and thus requires wide-arc but still partly-directional illumination ahead of him, e.g. in the manner of a hand-held torch or a vehicle-mounted headlamp. In another category of end-use the ultimate observer wishes to set up the device to provide high intensity all-round illumination, either temporarily as for instance at the scene of an accident or other emergency or on a more permanent basis as for instance in sporting arenas or other public concourse areas.